Materials which have been employed as the photoconductive materials of photoreceptors used in electrophotography include inorganic substances such as selenium, cadmium sulfide, zinc oxide, and the like.
The process of xerography, as disclosed by Carlson in U.S. Pat. No. 2,297,691, utilizes a photoconductive material comprising a support coated with a substance which can change its electric resistance depending upon the quantity of exposure which it receives during imagewise irradiation exposure, and which acts as an insulator under dark. In such a process, the photoconductive material is placed in the dark for an appropriate period of time for the purpose of dark adaptation, and then a uniform potential is created on its surface in the dark by a source of charge. Next, it is exposed to some form of irradiation having a pattern which possesses such an effect that the surface charge may be diminished, depending upon relative energy borne by each part of the pattern. The thus remaining surface charge or electrostatic latent image on the surface of the light-sensitive layer is converted into visible image by contact with an appropriate charge detecting and developing substance. Such a developing substance, referred to as a "toner", whether it is contained in an insulating liquid or in dry carrier, can adhere to the surface of the light-sensitive layer according to the electric charge pattern existing there. The toner attached to the electric charge pattern can be fixed thereon using known means, such as heat and/or pressure, or solvent vapors. On the other hand, it can be transferred onto a second support (e.g., paper, film, etc.). Similarly, it is possible to transfer the electrostatic latent image onto a second support and to develop it on the second support.
The process of xerography, wherein image is formed in such a manner as described above, is one basic type of imaging process. Fundamental characteristics required for photoreceptors in the electrophotography of this kind are as follows:
(1) The photoreceptor can be charged to create an appropriate value of potential in the dark.
(2) Leakage of charges created on the surface of a photoreceptor occurs in the dark only to a small extent.
(3) Selective discharging of the created charges takes place rapidly upon exposure to a pattern of light.
Conventionally used inorganic substances as described above have many advantages and at the same time, have various disadvantages. For instance, although selenium, which has so far been prevailingly used as a photoconductor, fully satisfies the above-described requirements (1) to (3), selenium photoreceptors suffer the disadvantages that (1) conditions for the production thereof are difficult to control, and therefore the production costs are high, (2) it lacks flexibility, and thus to shape it in a belt-form is difficult, and (3) it must be handled with great care because it is very sensitive to heat and mechanical impact. Cadmium sulfide and zinc oxide are each used for the production of photoreceptor in the form of dispersion in a resin binder. However, photoreceptors of this type do not well withstand repeated uses, due to mechanical deficiencies in smoothness, hardness, tensile strength, abrasion-resisting properties, and so forth.
In recent years, electrophotographic photoreceptors utilizing various organic substances have been proposed with the intention of overcoming the aforementioned defects inherent in the inorganic substances. At present some of these photoreceptors are put to practical use. For example, there is a photoreceptor comprising poly-N-vinylcarbazole and 2,4,7-trinitrofluorene-9-one (U.S. Pat. No. 3,484,237), a photoreceptor in which poly-N-vinylcarbazole is sensitized with a pyrylium salt series dye (published examined Japanese Patent Application No. 25658/73), a photoreceptor containing an organic pigment as a main component (Japanese Patent Application (OPI) No. 37543/72 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application")), a photoreceptor containing an eutectic complex of a dye and a resin as a main component (Japanese Patent Application (OPI) No. 10735/72) and so on. These photoreceptors have excellent characteristics and seem to be worthy of practical uses. However, taking into account various requirements for photoreceptors useful for electrophotography, fully satisfactory photoreceptors have not yet been obtained. Although the properties required for an excellent photoreceptor depend to some extent upon the end use thereof and the method employed for the production thereof, the use of an excellent photoconductive substance is generally the key to the production of photoreceptors possessing excellent characteristics.
Photoreceptors which utilize hydrazone compounds as photoconductive substances are described, for example, in U.S. Pat. No. 3,717,462, which corresponds to published examined Japanese Patent Application No. 8137/73, Japanese Patent Application (OPI) No. 59143/79, which corresponds to U.S. Pat. No. 4,150,987, Japanese Patent Application (OPI) Nos. 52063/80, 52064/80, and so on. In each of these cases, condensed polycyclic compounds or N-alkylamino substituted compounds of hydrazone are employed. However, these compounds have insufficient stabilities to oxidation with ozone generated by corona charging, light and heat, and unsatisfactory dark decay characteristics, and so on.
The hydrazone compounds having carbazole rings as described in Japanese Patent Application (OPI) No. 46760/80 are improved in their stability and dark decay characteristics to a considerable extent. However, the improvements in the stability and the dark decay characteristics are still insufficient, and, furthermore, these carbazole compounds suffer a serious disadvantage in that it is hard to obtain pure compounds because of difficulties in synthesizing them, and therefore production costs are very high.